Why? It sucks at large outdoor areas, for one. Highway 17 has a nasty draw distance, does not render a large body of water nicely, and really can't handle the terrain that IS displayed all that well (it's chunky looking, for example). Also, the map size limitations are a real problem. The maps in HL2 and Vampire are both MUCH smaller than what is present in Doom 3 and Far Cry. This drove me mad in both cases.
HL2 looks as good as it does mostly as a result of great texture work. Vampire often looks no better than an average Unreal Warfare game (or worse).
CryTek is a much better solution. Don't let the setting of Far Cry fool you, the engine is not just designed to render tropical islands. It can handle absolutely massive areas with surrounding indoor complexes all on ONE MAP.
CryTek's water system also seems to allow for large variation in size. It handles small pools as well as massive oceans. Source clearly has an issue when rendering large bodies of water. The ocean along Highway 17 is flat out UGLY looking.